Tracking apparatus



Dec. 12, 1950 J. H. LANcoR, JR

TRACKING APPARATUS 2 Sheets-Sheet 1 Filed Sept. 8, 1944 Dec. 12, 1950 J.H. LANcoR, JR 2,533,267

TRACKING APPARATUS Filed Sept. 8, 1944 2 Sheets-Sheet 2 gRNEY PatentedDec. 12, 1950 TRACKING APPARATUS Joseph H. Lancor, Jr., East Williston,N. Y., as-

signor to The Sperry Corporation, a corporation of Delaware ApplicationSeptember 8, 1944, Serial No. 553,191

'26 Claims. l,

This invention relates to radar tracking apparatus in which a beam ofradiant energy is conically scanned by rotation about an axis` and thephase of the modulation envelope of received energy reected from anobject is compared to the phase of a reference signal synchronized withsaid rotation to provide a measure of the displacement between theobject and the axis.

In apparatus of. this type, it has been found that the phase relationbetween the modulation envelope oi" received energy and the rotation ofthev beam varies with therange of the detected object. This phasevariation is due to transit time requiredfor energy to travel from thetransmitter to the object andl return. The beam may rotate through anappreciable angle during this transit. time thereby producing a phaseshift between the envelopev of. received energy and the position. of.the beam.

l This phase shift modiiies error signals repre-v senting the relativedisplacement of the tracking axis and the objectto an extent that itunbalances servomotor control. This is particularly evident.` in systemswhere the tracking axis is moved about two independent axes, usually inazimuth and elevation, to.M track the object. When the.- tracking errorysignals become unbalanced in@ such aV system, it causes the trackingaxis to approach the objectin a'spiral path.

important-object of the present invention is' to provide animprovedradar tracking system inl which compensation is madev forvariations inV the phase relation of received energy due to the rangeofthe detected object.

Another object of the invention isto provide animproved radar trackingsystem in which tracking signals are a true' measure of thedisplacement` between the tracking axis: and the deflected obj ect.

A further object of the invention is` to: provide' f an improvedautomatic radar tracking apparatus in which motors operate to moveA thetracking axis in a straight line path toward an object.

A still further objectof` the invention is to pro- In the drawings,

Fig. 1 is a schematic diagram of a radar tracking system embodying' oneform of the invention; and

Fig. 2 is a schematic diagram of a radar tracking system embodying amodied form of the invention.

In accordance with the invention, a directional transmitter radiates abeam of electromagnetic energy which is rotated about a tracking axis bycausing rotation of' the directional characteristic of the transmitterabout said tracking axis, This rotation of the directionalcharacteristic of the transmitter is used to develop a reference signalsynchronized with the rotation. In order to compensate for movement ofthe Ibeam duringA the time required for radiating energy to reach theobject and return, a phase shift is produced between themodulationenvelope of received energy reected from the object and thereference signal developed. by rotation of the directionalcharacteristic. This phase shift is preferably accomplishedautomatically lby controlling some phase shiftingV apparatus from anautomatic ranging circuit.

The phase. relation. between the modulation envelope of received energyand the reference signal as shifted isfcompared and utilized toprovide ameasure of the displacement between the tracking axis and the object.This measure of the displacement between the tracking axis and theobject may be used toindicate the tracking error or todirectly operatetracking servomotors adapted to move the tracking axis toward theobject.

Two arrangements for producing the relative phase shift between themodulation envelope.- of received energy and theV reference signal areshown in the accompanying drawings. In Fig. l the phase of the:reference signal is adjusted in accordance with the range: of theobject, whereasin Fig. 2 the-phase of the modulation envelope ofreceived energy is shifted according' to the range of the: object.

Referring iirst' to'Fig. l, there is shownan` antenna. II provided witha parabolic reiiector` I2 for producing a1 directional beam pattern` inthef form of a lobe 9 extending in a selected direction from the antennaIPI'. The relationship between the antenna I I/- and` the reiiector I2is selected so the lobe 9` is directedl at a small angle Withrespect toa central axis I0` of the parabola where'- by rotation ofthe antenna IIwith respect to the parabola I2 as by motor I3 causes the directionalcharacteristic'ofthe antenna to rotate about the*` Y motor I 3.

central axis I of the parabola I2, thereby conlcally scanning a radiatedbeam of electromagnetic energy about said axis, the axis of the parabolathen forming a tracking axis.

The antenna is supplied with high frequency radio energy from a suitabletransmitter Eli that is preferably controlled as by synchronizer i5 toperiodically generate short pulses of high irequency energy. Thetransmitter I4 is connected as by coaxial cable I6 to the antenna il.The entire antenna structure including the parabola i2 supported bybracket I'I mounted on a gear I 3 is adapted to be turned in azimuth bypinion IS. A gear 2l mounted on sleeve 22 extending through the gear I8and carrying a pinion 23 meshes with a gear 2d on shaft 25 rotates worm26 to turn sector 21 for rotating the entire antenna structure about anelevation axis. For convenience, and to avoid excessive rotating joints,etc., the coaxial cable I6 extends through sleeve 22 upwardly to theantenna Ii. Suitable rotating joints must, ofV course be provided topermit rotation of the antenna II by the motor I3.

That portion of radiated energy which is ree nected by an object ispreferably received by the same antenna II and transmitted by coaxialcable It to branch cable 3I and then to a T. R. box 32 that blocks thehigh power transmitted pulses from the transmitter If, but passes weakerreflected pulses to coaxial cable 33 which is connected to a suitablereceiver 34 that detects .the modulation of received energy. Such a T R.box has been described in the copending U. S. application .Serial No.406,494 in the name of Lyman et al. The output of the receiver it isconnected by leads 35 and 35 for supplying the modulation envelope ofreceived energy to a gate circuit 3l that is controlled from thesvnchronizer I5 in a manner to be described to pass only that portion ofreceived energy which corresponds to a selected object. The gate circuit3i may include an Eccles-Jordan or other known type of trigger circuithaving two stable conditions, and capable of being operated from one ofsaid conditions to the other by pulses applied to the leads 'i3 and isrespectively. Such circuits are described on page 206 of Theory andApplications of Electron Tubes by H. J. Reich, published by McGraw HillBook Company in 1939. As is well known to those skilled in the art, thevotages at certain points in an Eccles-Jordan circuit increase when thecircuit is operated to one stable condition, and decrease when it isoperated to the other condition. The gate circuit 3'! may also includean amplifier which is normally biased to cutoff, with its bias meansconnected to a terminal of the trigger circuit which goes positive whenthe lead 'I3 is energized, overcoming the c'uto'iT bias of theamplifier. Thus each pulse on the lead 'I3 turns on the amplier, andeach subsequent pulse on the lad 14 cuts it ci. The output of the gatecircuit 3l is supplied as by leads 38 and 39 to azimuth 'and elevationdemodulators il and 32, where the phase relation between the modulationenvelope of received energy and a signal developed by rotation of theantenna II is compared. The motor i3, which rotates the antenna II, alsodrives a'signal generator 43 which is shown as a two-phase generator forproducing voltages displaced in time phase by ninety electrical degrees.The generator 153 may be energized from a source of energy it which alsois connected to drive the One phase of the voltage from the generator t3is supplied as by leads it and lil 4 to the azimuth demodulator d iYwhereas the other phase of the generated voltage is supplied by leads#il and iii to the elevation demodulator d2? Each of these demodulatorsmay be conventional balanced demodulator circuits in which the cur rentconducted by the individual tubes arranged in a balanced circuit dependsupon the time phase relationship between voltages applied to theirplates and grids, respectively. ln this case, one of the appliedvoltages corresponds to the modu lation envelope of received energy,whereas the other is a reference signal from the generator 'lS. Theoutput of the azimuth demodulator, therefore, is a mzasure of the phasedisplacement between the modulation envelope of received energy and oneof the voltages from generator Since the beam of radiated energy iscon-V tinuously rotated about a tracking axis, the intensity of energyrefiected by an object displaced from that axis will harmonically vary,according to the position of the beam with respect to the object. Thisintensity wil vary at the same frequency at which the antenna is rotatedand will have a particular phase relation dependent upon the angularorientation of the object. if the reference signal supplied to theazimuth demodulator is selected to have maximum amplitude at the timethe beam from the antenna is directed horizontallygthe output of thedemodulator will provide a signal of a magnitude dependent upon theamount of horizontal dispiacenient between the tracking axis and theobject, and the polarity of this signal will determine whether thedisplacement is to the right or left of the axis. Similarly, byselecting the second voltage from the two-phase generator to havemaximum amplitude when the beam pattern ci the antenna is directedvertically, the'output of the demodulator l2 will p'rovide a variabemagnitude reversible polarity signal dependent upon the magnitude anddirection of the vertical displacementbween the axis and the object;

In order to compensate for the phase shif t in the modulation envelopeof received energy with respect to the reference `signals from thegenerator 3 that is caused by the angular rotation of the antenna Iiduring the transit time of energy to thel obiect and return there isshown' in Fig. l apparatus for shifting the phase of 'the referencesignal according to the range of the object. This may be accomplished byturning shaft 5I which carries pinion 52 meshing with gear 53 forrotating the casing of the signal generator 43. This changes the phaser;laticn of the two-phase voltage generated by the generator 43 vwithrespect to the anguar position of the directional pattern of theantenna. The angle of'this phase shift is determined by the angle whosetangent is a function of the ratio of the range to th; spin frequency ofthe antenna which may be expressed as i mit movement of the generatorwithout disturbing the mechanical connections.

To automatically adjust the phase of the reference signals, there isshown an automatic range circuit which controls motor to operate throughtraduction gear 156 'to control the sha-ft 5| `'forshifting the .casingof the reference Sgen eratora. This.automatic range circuit :includes aydelay multivibrator indicated generally at .5l which -is .triggeredsynchronously with the transmitted pulse rfrom synchronizer |5. -Thevvsyn-- chronizer :i 5 is coupled .bycondenser 58 :and lead 59Ato-.onegrid:E39-of .a-doubleftriode tube ii. The double tr-iode 6| hasthe `.electrodes `.of its two electron-.discharge vpaths Lintaconventional multi vibrator .circuit to produce fan output pulse on lead,6.2 yhaving .an adjustable phaserelation with respect to thetransmitted pulse. The A.time rdelay .between the transmitted pulse andthefpolse output 4of .-.the multivibrator, .appearing on lead .52 isdetermined by ,the position of .slider .fell on potentiometer .65 `thatcontrols the .bias .applied .to grid .59 of :the ,double .triode ..6|,Athereby icontrolling .the time -delay .between the application of thetrigger pulse -to .the grid 5,5 and the 4output pulse .at lead v52 ,inwell known manner. The position .of the .potentiometer .slider t4controlled by shaft TU 'that is also driven from the range motor 55 thatyis controlled automatically by a circuit .to 4be described.

4The output pulse of Vlead B2 .is coupled as by condenser "H to one sideof a delay line designated generally at 2 which includes suitableindnctance, capacitance and resistance arranged to produce la pulse atthe opposite 'end of the line vdisplaced in time phase with that appliedto the line by a short interval preferably yof 'the fsa-Ine order ofAmagnitude as the time duration of each transmitted pulse of energy. Thepulse from condenser fil "is vcoupled as by lead l Ito the gate circuitVtl land vthe delayed pulse from therother end `o'f the line "issupplied by lead to the gate circuit 3l. The gate circuit is so arrangedthat the pulse on 'the lead :causes the .gate .circuit to :pass the.energy Sironi .the receiver X34 to the demodulators :di :and ft2.whereas .the .delayed pulse .fromthe 'lead :1A renders .the gate,circuit .non-.conductive A,until such .time as 'another pulse fis`applied bfyiglead With this arrangement, the .gate lcircuit rblocks allsignals .from the receiver except those .received during the timeinterval between :the .two pulses from the .delaylline 12. Since'the.phase frelatifm of the pulse from the multivibrator 15'@ is 'con'-trolled by "the .position :of lslider 164, the time interval lduringwhich the `gate circuit 'conducts energy from the vreceivertothedeniodulators .is vadjustable .to correspond to .the .time .atwhich energy vfrom a selected object `is received.

line 'l2 are applied b yleads il andl' to grids T59 and. .3| of .double.triode 82 having .its v.electrodes arranged in a balancedcathodeiollower circuit so ,output .pulses ,appearing across cathode`resistors 53 and .85 are coupled through condensers 85 and 'B5 acrossresistors B] .and 'S8 ltobe .applied to grids .9| and 92 of double.triode The double 'triode S3 has vits electrodes arranged in a balancedampliner circuit to 'form a `range error detector with its plates Si andS5 coupledithrough condensers '9S and "El "respectively to lead from thereceiver. Since cathodes 'im and It? oi the double triode 93 aregrounded and lead '3E from the receiver is grounded, `it `will beapparent that the modulation envelope of received energy is applied tothe plates of the range error detector circuit.

Dependent upon the 'time relationfof 'the =en velope of received pulseswith respect to 4the spaced pulses applied to grids 49| and i812respectively, one 4side :or 'the other :of the :double ftriode 93 willconduct more current, according to whether fthe time relation of `thereceived signal is advanced or retarded with `respect to 'the totalpulses trom fthe Ldelay line lf2.. With this .anrangement, -a:differential voltage .iS .developed across load .resistors Vand .|85`connected `to plates .8.5 fand S5 respectively. The :polarity of thisY'voltage will depend upon the direction `of the range error asdetermined `by the settingfof potentiometer G5, whereas the magnitudewill depend Aupon the amount of the range error. This difierentialvoltage is applied `to grids |56 yand Hl? -of .double triode |08Yconnected as a direct current ampliner. .Coupling resistors |09 and-for :the `plates 9@ and .95 are connected to Y controlgrids JDS and |91.respectiyly, to control the Ipotential of plate ||2 of the 4doubletriode .'.s acting .as a-difference .detector according .to the .rangeerror signal from Vthe range detector.

Thefunction .of tube 58 acting as `a difference detector :is .to accepttwo `signal input voltages from rangeerror detector tube -93 .anditodeliver a-single `difference output voltage from tube `Hi8 to -tube .l|74. li the input signals to grids .|51 and |556 .of `tube 1&8 `areequal, .the output from tube ISS to tube H4 will .be zero. However, .ifthe .input signals .to .the aforesaidgrids are nnedual, a .dierencevoltage will .be applied .to tube Ht from plate .|53 `to grid .|015 oftube |45. .diierence voltage `is 4.achieved through .the `of the common`.Cathode .resistor |58. 'The outpntroin .section |159 `of -amplier |98,appears across the cathode resistor, giving a degenerative or.differential action .thereacross Since cathode is tied ,in at a high.negative .potential thro-ugh resistor 115.8, ,plate '116| which is .atground will be sufciently positive to allow for operation opoi-tion |59of the tube.

ihisrange error signal is then directly applied torgiid H3 of .doubletriode I|4,`havingits electrodes `arranged in a .differential ampliiiercircuit .toprovide a differential voltage across load resistors |15 and.116., one side off which .applie-:lfoireetlylto grids Ul and U3, and`the other side of whichisapplied directly togrids t9 and |.2| .of.double triodes |22 and |23 arranged ina full-wave balance modulatorcircuit of conventional design, .that .is .supplied .from a .suitablesource I|25 of .alternating voltage .through .transformer IE6.. The.output of the full-w ve balanced modulator .is .a variable magnitude.reversible .phase .alternating `voltage that is .coupled bytransformer31 togrids 32..and |33 of double triode .itljhavingits yplates |35 and.|35 controlling opposite halves of .aeld winding of .the alternatingYcurrent .range motor .55 that 'has its other vwinding L39 `connected tothe yalternating voltage source |25.

`Since `the phase and magnitude `of the lvoltage applied to the winding38 Ais dependent vupon the relativefphaseoi the doublepulses fromthedelay line 12 .and thepulses from receiver 34, as determined by rangeerror detector tube S3, therange motor is caused to 'rotate in adirection and atfa speeddependent upon :the range error signal to move.thepotentiometer slider inea direction to shift the time relation ofthe .double pulses to coincide-with the .received pulses. -When anequilibrium :condition is reached, the double pulses from the :delay.line 'i2 `will be arranged symmetrically oniopposite sidesfof receivedpulses from :the :receiver .34, thereby reducing vthe differentialsignals from -the range error .detector to zero and at the same timeystopping the :range 15 motor 55.

' nal.

In order to stabilize the range motor 55, a velocity damping signal isprovided by generator I4! which is driven by gear |42 from the rangemotor and has one winding |43 connected to the alternating currentsource and another winding |44 connected to the other grid |46 of thedouble triode H4 to introduce a degenerative signal in the diierentialamplifier circuit of the tube Htl corresponding to the velocity of themotor 55.

With the arrangement described, the phase relation between themodulation envelope of received energy and the reference signal fromgenerator i3 is adjusted in accordance with the range of a selectedobject. This adjustment is accomplished by shifting the phase of thegenerator reference signals. Since the phase relation is adjustedaccording to the range of the object, the output of the azimuth andelevation demodulators provide accurate measures of the azimuth andelevation displacements of the detected object with respect to thetracking axis. These azimuth and elevation displacement signals maycontrol suitable azimuth and elevation indicators 15| and |52 toindicate azimuth and elevation tracking errors.

In order to provide an automatic tracking system, these azimuth andelevation error signals may be used to directly operate azimuth andelevation servomotors |53 and |54. motor drives through shaft and gearI9 to rotatethe antenna Il in azimuth in a direction to reduce theazimuth tracking error signal. The elevation motor |54 drives throughshaft |56 and pinion |51 to rotate gear 2| for turning the antenna Ilabout an elevation axis in a direction to reduce the elevation errorsignal. Since the azimuth and elevation error signals have beencorrected for errors due to the range of the object, the azimuth andelevation motors will direct the axis of the antenna toward the detectedobject. By controlling the speeds of these motors according to themagnitudes of the respective error signals, the tracking axis will bemoved in a straight line toward the object, since theA error signalshave been corrected for error due to the range of the object.

The circuit shown in Fig. 1 illustrates one embodiment of the inventionin which the phase of the generated reference signal is shifted. Thecircuit shown in Fig. 2 illustrates one arrangement for adjusting thephase relation between the modulation envelope of received energy andthe reference signal by shifting the phase of the received modulationenvelope. This circuit is substantially the same as that shown in Fig. 1and identical reference numerals have been applied to correspondingelements.

'In the circuit shown in Fig. 2, the reference generatcr 43 has itscasing ixed with respect to the antenna I to provide reference signalshav-l ing a predetermined phase relation to the rotation of the antenna.The range motor 55 drives Y through reduction gear 5B to position shaft5| which functions as did shaft 5| in Fig. 1 to adjust the phaserelation between the modulation en- The azimuth energy that is passed bythe gate circuit 31 and corresponds to the modulation envelope of energyreected from a selected object, since the gate circuit is controlled topass only that energy which is received from an object at a selectedrange as determined by the position of range motor 55. Since the rangemotor 55 operates to vary the resistance of variable resistor |12, itcontrols the time constant and thereby the effect of the phaseshiftingnetwork to shift the phase of the modulation envelope ofreceived energy an amount corresponding to the range of the detectedobject.

In this modied form of the invention, the outputs of the azimuth andelevationl demodulators 4| and 42 provide accurate measures of the azi-Y muth and elevation tracking displacement errors velope of receivedenergy and the reference sig- However, shaft 5| operates through shaft1G to control a slider |1|V on a variable resistor |12 thatcooperateswith condenser |13 to form a phase shifting network interposedbetween the output of the gate circuit 31 and the inputs of the azimuthand elevation demodulators 4| and 42. The phase shifting network isselected to adjust the phase of the modulation envelope of receivedwhich may be used to indicate tracking errors or to control theservomotors for automatically moving the tracking axis 'toward theobject. Since the phase displacement between the modulation envelope ofreceived energy and the reference signal is adjusted according to therange of the object, it is compensated for errors due to the transittime of energy to and from the detected object. The tracking errorsignals from the azimuth and elevation demodulators are thereforeaccurate signals, independent of errors due to changes in the range ofthe object.

Since many changes could be made in the above construction and manyapparently widely differentembodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is: y

1. An object detecting system comprising directional means for radiatingelectromagnetic energy having its directional characteristic rotatedabout an axis, means for receiving that portion of energy reflected froman object, means controlled by said rotation for developing a referencesignal, and means for producing a phase shift between the modulationenvelope of received energy and said reference signal according to therange of a detectedY object. y

2. An object detecting system comprising directional means for radiatingelectromagnetic energy having its directional characteristic rotatedabout an axis, means for receiving that portion of energy reflected froman object, means controlled by said rotation for developing a referencesignal, an

automatic range circuit for providing a measure ofthe range of adetected object, and means controlled by said range circuit forproducing a phaseA shift between the modulation envelope of relceivedenergy and said reference signal according to the range of said object.Y 3. An object detecting system comprising directional means forradiating electromagneticN energy having its directional characteristicrotated about an axis, means for receiving that portion of energyreflected from an object, means controlled by said rotation fordeveloping a reference signal, an automatic range circuit for providinga measure of the range of a detectedl ing its directional characteristicrotated about an axis, means for receiving that portion of energyreflected from an object, means controlled by said rotation forproducing a reference signal, means for producing a phase shift of themodulation envelope of received energy according to the range of adetected object, and means responsive to the relative phase of saidphase shifted envelope and said reference signal for providing a measureof the relative displacement between said axis and said object.

16. In a tracking system having a' reference phase generator and adirectional antenna for radiating electromagnetic energy, the methodcomprising the steps whereby the directional characteristic of saidantenna is referentially rotated about an axis and having the phasalrelation of the modulation envelope of received energy reflected from anobject to said rotation provides a measure of therelative displacementbetween said axis and said object, the further steps in said method omeasuring said displacement and increasingly adjusting the phaserelation between said envelope and said rotation according to theincreasing range of the object.

17. In a tracking system having a directional antenna for radiatingelectromagnetic energy in which the directional characteristic of saidantenna is rotated about an axis, the method of measuring the relativedisplacement between said axis and a detected object which comprisesdetecting thel modulation envelope of received energy reflected from anobject, producing a reference signal according to the rotation of saiddirectional characteristic, producing a4 phase shift between saidenvelope and said signal according to the range of said object, and thenmeasuring the phase relation between said envelope and said signal.

18. In a tracking system having a directional antenna for radiatingelectromagnetic energy in which the directional characteristic of saidantenna is rotated about an axis, the method of measuring the relativedisplacement between said axis and a detected object which comprisesdetecting the modulation envelope of received energy reflected from anobject, producing a reference signal according to the rotation of saiddirectional characteristic, producing a phase shift of said referencesignal according to the range of said object, and then measuring thephase relation between said envelope and said signal.

19. In a tracking system having a directional antenna for radiatingelectromagnetic energy in which the directional characteristic of saidantenna is rotated about anaxis, the method of measuring the relativedisplacement between said axis and a selected object which comprisesdetecting the modulation envelope Vof received energy reflected from anobject, producing a reference signal according to the rotation of saiddirectional characteristic, producing a phase shift of said envelopeaccording to the range of said object, and then measuring the phaserelation between said envelope and said signal.

20. In a tracking system in which a beam of electromagnetic energy isreferentially rotated yas to phase about an axis and the phase relationbetween energy reected from an object and said referential rotation iscompared, means for increasingly adjusting said phase relation accordingto the increasing range of said object to compensate for said phasedifference :according to said comparison.

21. An object detecting system comprising rotating means for irradiatingsaid object with radio energy, means for receiving modulated reflectionsfrom said object, a reference signal generator controlled by saidfirst-named means and means for shifting the phase between `saidmodulation envelope and said reference signal according to the distanceto said object.

22. An object detecting system comprising rotating scanning means forirradiating said object, means for receiving modulated reflections fromsaid object, phase comparator means controlled by said first-named meansand means for aligning the phase between said mod-ulation envelope andsaid comparator according to the range of said irradiated obj ect.

23. An object detecting system comprising a rotating antenna forirradiating said object with electromagnetic energy, means receivingmodulated reflections from said object, aY reference signal generatorcontrolled by said rotating antenna and means for correcting the phasediscrepancy between said modulation envelope and said reference signalaccording to the distance to said object.

24. In an object detecting system, a phase correction device comprisinga rotating scanner irradiating an object with radio energy, means forreceiving modulated reflections from said object, a reference signalgenerator lcontrolled by said rotating scanner and means for shiftingthe phase between said modulation and said reference signal so as tocorrect the phase discrepancy therebetween according to the distancefrom said scanner to said object.

25. In an object detecting system, a phase synchronizer comprisingrotating directive means irradiating an object with radio energy, meansfor receiving modulated reflections from said object, a referencegenerator controlled by said rotating directive means given a referencesignal, and means synchronizing the phase between said modulatedreflectors and said reference signal according to the range between saiddirector and said object.

26. In an object detecting system utilizing an antenna rotating about anaxis at a predetermined spin frequency, a method of determining thedisplacement between said axis of rotation and said object comprisingthe steps 0f irradiating said object with electromagnetic energy,receiving modulated reflections therefrom and determining thedisplacement between said object and said axis of rotation according totime ofk transit of the electromagnetic energy to said object and thespin frequency of said rotating antenna.

JOSEPH H. LANcoR, Jn.

REFERENCES CITED The following references are of record in theV le ofthis patent: Y

UNITED STATES PATENTS

